1. Field of the Invention
This invention relates to the field of video systems utilizing a liquid crystal display (LCD) or liquid crystal on silicon (LCOS), and in particular, to a driver circuit for improving brightness control in LCOS/LCD projection systems.
2. Description of Related Art
Liquid crystal on silicon (LCOS) can be thought of as one large liquid crystal formed on a silicon wafer. The silicon wafer is divided into an incremental array of tiny plate electrodes. A tiny incremental region of the liquid crystal is influenced by the electric field generated by each tiny plate and the common plate. Each such tiny plate and corresponding liquid crystal region are together referred to as a cell of the imager. Each cell corresponds to an individually controllable pixel. A common plate electrode is disposed on the other side of the liquid crystal. The drive voltages are supplied to plate electrodes on each side of the LCOS array. Each cell, or pixel, remains lighted with the same intensity until the input signal is changed, thus acting as a sample and hold (so long as the voltage is maintained, the pixel brightness does not decay). Each set of common and variable plate electrodes forms an imager. One imager is typically provided for each color, in this case, one imager each for red, green and blue.
It is typical to drive the imager of an LCOS display with a frame-doubled signal to avoid 30 Hz flicker, by sending first a normal frame in which the voltage at the electrodes associated with each cell is positive with respect to the voltage at the common electrode (positive picture) and then an inverted frame in which the voltage at the electrodes associated with each cell is negative with respect to the voltage at the common electrode (negative picture) in response to a given input picture. The generation of positive and negative pictures ensures that each pixel will be written with a positive electric field followed by a negative electric field. The resulting drive field has a zero DC component, which is necessary to avoid image sticking, and ultimately, permanent degradation of the imager. It has been determined that the human eye responds to the average value of the brightness of the pixels produced by these positive and negative pictures so long as the frame rate is above 120 Hertz.
The present state of the art in LCOS requires the adjustment of the common-mode electrode voltage, denoted VITO, to be precisely between the positive and negative field drive for the LCOS. The subscript ITO refers to the material indium tin oxide. The average balance is necessary in order to minimize flicker, as well as to prevent a phenomenon known as image sticking.
In the current art, the LCOS drive cell looks much like a conventional Active Matrix LCD driver. This does not work well, due to the various artifacts discussed in the literature. The main causes are parasitic capacitance cross-talk, residual voltage in the LC cell, and voltage droop of the LC, due to ionic leakage and bulk resistivity of the LC material. Mainly, this has been solved by: 1. Increasing the cell capacitance (limited by physical area), 2. Changing to higher resistivity LC materials (limits flexibility and response time), 3. Increasing the frame scan rate to more than 60 Hz (expensive, and costs more bandwidth), 4. Strongly controlling the temperature of the device, to maintain high voltage holding ratio (VHR). All these detriments also have an impact on the ability to control brightness in a liquid crystal or LCOS display.
The prior method for implementing brightness control in a display consists of performing a mathematical add function on the digital data prior to applying it to the display. The problem with this method is that depth of color is greatly impacted, since all of the brightness range must be accommodated in the pre-processing. Moreover, there is no way to blank the display without destroying the data in the typical architecture.